一.1 oracle 如何预估将要创建的索引的大小
oracle 提供了2种可以预估将要创建的索引大小的办法：
① 利用包 Dbms_space.create_index_cost 直接得到
② 利用11g新特性 Note raised when explain plan for create index
下边分别举例说明。
一.2 环境说明
[oracle@rhel6_lhr ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.3.0 Production on Fri Dec 26 15:58:06 2014
Copyright (c) 1982, 2011, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, Automatic Storage Management, OLAP, Data Mining
and Real Application Testing options
SQL> create table test_index_size as select * from dba_objects;
Table created.
SQL> EXEC DBMS_STATS.gather_table_stats(ownname => 'SYS',tabname => 'TEST_INDEX_SIZE');
PL/SQL procedure successfully completed.
一.3 第一种 Dbms_space.create_index_cost
脚本：
declare
l_index_ddl varchar2(1000);
l_used_bytes number;
l_allocated_bytes number;
begin
dbms_space.create_index_cost(ddl => 'create index idx_t on sys.test_index_size(object_id) ',
used_bytes => l_used_bytes,
alloc_bytes => l_allocated_bytes);
dbms_output.put_line('used= ' || l_used_bytes || 'bytes' ||
' allocated= ' || l_allocated_bytes || 'bytes');
end;
/
实验：
SQL> set serveroutput on
SQL> declare
2 l_index_ddl varchar2(1000);
3 l_used_bytes number;
4 l_allocated_bytes number;
5 begin
6 dbms_space.create_index_cost(ddl => 'create index idx_t on sys.test_index_size(object_id) ',
7 used_bytes => l_used_bytes,
8 alloc_bytes => l_allocated_bytes);
9 dbms_output.put_line('used= ' || l_used_bytes || 'bytes' ||
10 ' allocated= ' || l_allocated_bytes || 'bytes');
11 end;
12 /
used= 383105bytes allocated= 2097152bytes
PL/SQL procedure successfully completed.
SQL>
PL/SQL 过程已成功完成。
说明: used_bytes 给出索引数据实际表现的字节数。
allocated 是当实际创建索引时在表空间中实际占用的字节数。
一.4 第二种11g新特性:Note raised when explain plan for create index
这是一个挺实用的小特性，在11g r2中使用explain plan for create index时Oracle会提示评估的索引大小(estimated index size)了：
SQL> set linesize 200 pagesize 1400;
SQL> explain plan for create index idx_t on sys.test_index_size(object_id) ;
Explained.
SQL> select * from table(dbms_xplan.display());
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------
Plan hash value: 32582980
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | CREATE INDEX STATEMENT | | 76621 | 374K| 350 (1)| 00:00:05 |
| 1 | INDEX BUILD NON UNIQUE| IDX_T | | | | |
| 2 | SORT CREATE INDEX | | 76621 | 374K| | |
| 3 | INDEX FAST FULL SCAN| IDX_T | | | | |
--------------------------------------------------------------------------------
Note
-----
- estimated index size: 2097K bytes
14 rows selected.
一.5 创建真实索引
SQL> create index idx_t on sys.test_index_size(object_id) ;
Index created.
SQL> analyze index IDX_T validate structure;
Index analyzed.
SQL> select bytes from dba_segments where segment_name='IDX_T';
BYTES
----------
2097152
可以看到2种办法给出的索引评估大小与实际索引占用空间差别不大，但这里有个前提条件就是预估索引大小之前必须对表进行分析过。